Devices for measuring various electrical parameters, such as voltage, current and resistance are in common use. A typical example is a multimeter, which generally can measure AC or direct current (“DC”) voltage and current as well as resistance. Multimeters typically include a set of test leads that are adapted to be connected to a pair of test points. The test leads are coupled to an internal amplifier, which drives circuitry for providing information to a read-out device such as an analog meter or a digital display. A typical amplifier circuit 10 is shown in FIG. 1. The amplifier circuit 10 includes an operational amplifier 12 having a non-inverting input connected to ground. An inverting input of the operational amplifier 12 forms a summing junction that is connected to one input terminal 14 of the multimeter through an input resistor 18 and to an output of the operational amplifier 12 through a feedback resistor 20. Another input terminal 24 is connected to ground. As is well-known in the art, the gain of the amplifier 12 is set by the ratio of the resistance of the feedback resistor 20 to the resistance of the input resistor 18.
It is generally desirable for a multimeter to have a very high input impedance. For this reason, the input resistor 18 typically has a very high resistance, such as 1 MΩ. The resistance of the feedback resistor 20 is typically much lower, such as 10 ka Therefore, the gain of the amplifier 12 is low. Using the examples given (1 MΩ input resistor 18 and 10 kΩ feedback resistor 20), the gain of the amplifier 12 would be 0.01.
The output of the amplifier 12 is then applied to a high gain amplifier 30. The low gain of the amplifier 12 attenuates the signal to be measured, but, unfortunately, it does not significantly attenuate noise that may be present in the signal or present in the multimeter. Therefore, when the output of the amplifier 12 is boosted by the high gain amplifier 30, the signal-to-noise ratio of the measured signal can be very low.
Another “front end” amplifier circuit 40 that is conventionally used in multimeters is shown in FIG. 2. The circuit 40 also uses an operational amplifier 44 configured as a voltage-follower with its output connected to the inverting input of the amplifier 44. The signal to be measured is applied to the non-inverting input of the amplifier 44 through a resistor 46. The amplifier 44 has a very high input impedance between its inverting and non-inverting inputs. However, to fix the input impedance at a constant, controllable value, an input resistor 48 may be used between the input terminal 14 and ground. The input resistor 48 may have a high resistance value, such as 1 MΩ. Unfortunately, stray capacitance and input capacitance of the amplifier 44, both of which are represented by a capacitor C, forms a low-pass filter with the resistor 46. This low-pass filter can limit the AC response of the amplifier circuit 40. Although the circuit 40 can still be used in the measurement of the voltage and current of DC signals, the low-pass filter can result in measurement errors for AC signals, particularly if the AC signals have a high frequency.
There is therefore a need for a circuit for amplifying a signal to be measured in a manner that results in a high signal-to-noise ratio, a high, stable input impedance and good high frequency performance.